What does a no-vig calculator do?
A no-vig calculator removes the bookmaker’s margin from a complete set of odds. It gives fair odds and market-implied probabilities using proportional normalization.
Method and assumptions
The no-vig formula
For decimal odds d, implied probability is 1 ÷ d. Add these probabilities across every outcome to get the book total S. Fair probability for outcome i is (1 ÷ dᵢ) ÷ S; fair decimal odds are 1 ÷ fair probability. Overround is (S − 1) × 100%. This proportional method preserves the ratio between the original implied probabilities.
- Convert every outcome to decimal odds.
- Sum the implied probabilities: S = Σ(1 ÷ d).
- Divide each implied probability by S.
- Invert each normalized probability to get fair decimal odds.
Example: 1.91 on both sides
Two outcomes priced at 1.91 each imply 52.36% per outcome, or 104.71% in total. The overround is 4.71%. Dividing each implied probability by the total gives 50% per outcome: fair decimal odds of 2.00, or American odds of +100. Values shown are rounded; calculations use full precision.
Compare with a prediction-market price
A no-vig estimate of 60% corresponds to a reference price of $0.60 for a contract that pays $1 if the outcome occurs. It is a comparison point, not a recommendation to trade. Match the exact event and resolution rules, then account for fees, the bid–ask spread, and slippage before evaluating any difference.
- Use odds from the same source and time to estimate that market’s margin.
- Do not omit the draw or other possible outcomes.
- Prices from multiple venues can have different resolution and execution conditions.
Assumptions and limits
The model assumes a complete set of mutually exclusive outcomes and allocates margin proportionally. It does not adjust for favorite–longshot bias, model push or void probabilities separately, or establish a true probability. Other methods of removing vig can produce different estimates.
- Decimal inputs must be greater than 1 and no more than 1,000,000.
- American inputs must be at least +100 or at most −100 and convert into the supported decimal range.
- Totals below 100% are flagged as underround. Missing outcomes or stale quotes can cause this; it is not proof of executable arbitrage.
- Unrounded probabilities sum to 100%; displayed values can differ slightly because of rounding.